Field observations suggest that the more virulent dengue-2 genotype of SE Asian origin is competitively displacing the indigenous, American strain of dengue-2. This proposal shall determine whether these differences in viral fitness stem from their physiological interaction with the mosquito vector. It may be that the SE Asian genotype is better adapted for vector transmission than its native counterpart. To evaluate. This hypothesis, we shall: 1) compare the ability of SE Asian and native genotypes of dengue-2 to infect and been transmitted by vector mosquitoes; 2) identify physiological barriers to vector infection, dissemination, and transmission, and 3) compare the relative fitness of both viral genotypes in diverse mosquito populations. This integration of physiological and genetic approaches will help facilitate our understanding of viral-vector interactions.